Lithographic methods are often used for producing deterministic microstructures. In this case, a radiation-sensitive polymer layer (resist) is exposed with a laterally varying dose distribution of a specific type of radiation. The introduced radiation dose alters the polymer properties in such a way that, in a subsequent development step, selectively only the irradiated (positive resist) or only the unirradiated (negative resist) regions of the polymer layer are dissolved and thus removed.
Lithography systems used in microelectronics and microsystems engineering often use a so-called mask aligner (mask positioner) that is a device for exactly positioning photomasks. In this case, the light from a beam source having the shortest possible wavelength, for example, from a high-pressure mercury lamp, is collected with the aid of an illumination unit and used for illuminating a prefabricated photomask. Said photomask contains the structures to be exposed into the radiation-sensitive polymer layer in the form of transparent and non-transparent regions which are introduced e.g. into a thin chromium layer on a mask substrate. In the transparent regions, the chromium layer is selectively removed for this purpose. With such a photomask, the illumination light is correspondingly modulated upon passing through the photomask. The polymer layer to be exposed is situated on a substrate that is brought into direct contact with the photomask (so-called contact printing), or is situated at a small distance of a few micrometers from the photomask (so-called proximity printing).
In recent years, specific mask aligner based lithography methods have been developed in which the structure is no longer copied from the mask into the photoresist by simple shadow casting, rather diffraction effects at specifically calculated photomask structures are used in a targeted manner for resolution enhancement in the exposure of the photoresist. Such a method is known, for example, from the document T. Weichelt, U. Vogler, L. Stuerzenbecher, R. Voelkel, U. D. Zeitner, “Resolution enhancement for advanced mask aligner lithography using phase-shifting photomasks”, Optics Express 22 (2014), 16310-16321. Such a method is also referred to as diffraction lithography.
Conventional illumination using high-pressure Hg lamps is encountering limits particularly in diffraction lithography. The diffraction-lithographic methods typically require mask illumination with very low residual divergence of the illumination angles. In conventional illumination this can be realized only by means of an aperture stop having a very small opening in the illumination beam path. As a result of the small stop opening, however, the available light power decreases to such a great extent that impracticably long exposure times are the consequence. On account of the properties of high-pressure mercury lamps (thermal beam source having a source volume in the range of mm3) this problem is virtually insurmountable.
Alternatively, a laser can be used as beam source, a significantly higher degree of collimation of the illumination being achievable with said laser. However, the high coherence of lasers in a conventional illumination beam path of the mask aligner leads to the occurrence of spatially high-frequency interference patterns (so-called speckles) which significantly impair the homogeneity of the mask illumination locally and lead to unusable exposure results.